Thermal polycondensation of aspartic acid and aspartic acid precursors, such as maleic acid plus ammonia, to produce polysuccinimide, which is then converted to polyaspartate by mild alkaline hydrolysis (see scheme below), has been the subject of commercial research and development for more than two decades. Much of this effort is summarized in U.S. Pat. Nos. 5,981,691 and 6,495,658 to Sikes and coworkers (1999, 2002) and in copending U.S. application Ser. No. 10/431,124 (PCT Application Ser. No. US03/14312) by Sikes, which are incorporated herein by reference.

Generally, the polyaspartates formed by polysuccinimide ring opening comprise both D- and L-aspartate residues, and have amide linkages through both the α and β carboxylic groups, as shown, regardless of the stereochemistry of the aspartic acid (or aspartic acid precursor) monomers.
The principal commercial routes to polysuccinimides and polyaspartates now in use include the maleic acid/ammonia method and direct thermal conversion of aspartic acid monomer, as discussed in the above-referenced cases. These routes generate molecules of relatively low molecular weight (several thousand daltons), often having significant branching (branch points as frequent or more frequently as every tenth residue on average) and often ranging in color from light tan to a darker reddish color. Such molecules have been commercialized in specialty detergents and as environmentally friendly oilfield additives. However, many if not most markets often require or at least favor higher molecular weight polymers, if available.
Methods for preparing higher molecular weight polysuccinimides via acid catalysis have been developed. See, for example, Fox and Harada, 1962, U.S. Pat. No. 3,052,655; Neri et al., J. Med. Chem. 16, 893–897 (1973). The catalysts are typically phosphoric acid or polyphosphoric acid, employed at up to 50–65% by weight, relative to the aspartic acid monomer. In general, an admixture or paste of aspartic acid (powder or crystal) with the phosphoric catalyst is formed, then thermally polymerized, e.g. at 200 ° C. for 2–4 hours, to produce the polysuccinimide. Molecules in the range of 30,000 and somewhat higher are readily achievable via these methods. Color formation also tends to be suppressed under these conditions, resulting in polymers of favorable, off-white color.
However, the amount of phosphoric or polyphosphoric catalyst required for optimal reaction by this method can be very high. It is probable that the large quantities are needed to maintain acidic conditions in the reaction, preventing thermal decomposition of amino termini, which leads to chain termination. In addition, the hygroscopic phosphoric acid probably serves to remove water, a byproduct of the condensation reaction, thus promoting the reaction. Upon continued heating, this captured water could be vacated to the atmosphere (or the vapor space in the reactor), restoring the hygroscopic tendency of the phosphoric compound.
Later improvements in conjunction with this approach have included a two-stage reaction, in which an intermediate product is mechanically communited, followed by further condensation (Knebel et al., 1992, U.S. Pat. No. 5,141,062); addition of processing aids such as zeolites, sulfates, and bisulfates (Adler et al., 1995, U.S. Pat. No. 5,457,176); use of solvents such as diphenyl ether for azeotropic dehydration (Nagatomo et al. 1996, U.S. Pat. No. 5,484,945); addition of surfactants to further disperse and promote mixing of the reactants, intermediates, and products (Chou, 1999, U.S. Pat. No. 5,856,427); the use of super polyphosphoric acid (Tang, 1999, U.S. Pat. No. 5,929,198), and the use of cyclic propylene carbonate as a solvent (Mazo et al., 2001, U.S. Pat. No. 6,197,897). Knebel et al., cited above, reported non-crosslinked polysuccinimides having molecular weights greater than 100,000, using a process that requires interrupting polymerization, grinding (comminuting) a solid intermediate, and then continuing the polymerization. The molecular weights were determined by viscometric measurements, which tend to give higher values than gel permeation chromatography (GPC) measurements. Morphology of the products (i.e. linear or branched) was not reported.
Uenaka el al. (1997, EP 0791616 A1) employed triphenyl phosphite or tributyl phosphite as catalysts, along with organic solvents including acetone, mesitylene, and sulfolane. These authors reported production of polysuccinimides up to Mw 75,000.
Irizato et al. (2001, JP 2001302794 A2) taught a modification of traditional phosphoric acid catalyzed thermal polymerization of aspartic acid, in which a fluid mixture of aspartic acid in water, methanol, ethanol, or a combination of these solvents, along with the phosphoric catalyst, is formed. The mixture is atomized and spray-dried and simultaneously thermally polymerized. The polysuccinimides so produced were reported to range in Mw from 3,000 to 200,000.
Other approaches to generating polysuccinimides of higher Mw have employed various non-phosphoric acid catalysts. For example, gaseous carbon dioxide, which purges the reactor to carry away the water of condensation and also provides a mildly acidic environment, has been used to increase the Mw of product polysuccinimides (Kalota and Martin, 1994, U.S. Pat. No. 5,329,020). There is also a report of the use of benzene sulfonic acid as a catalyst during thermal production of higher Mw polysuccinimides (Wang, 2000, CN patent 1267673). In related approaches, Kato et al. (1999, 2000) polymerized aspartic acid in the presence of substoichiometric amounts of gaseous or aqueous HCl. If, in addition, an aprotic solvent was used (JP 11-240947 A2), these authors reported production of polysuccinimides up to Mw 60,000. In the absence of solvents, in other words by use of aqueous or gaseous HCl alone to provide acidic conditions, the Mw of the product polysuccinimides was reported to range up to 30,000 (JP 2000-169577 A2).
Other non-phosphoric methods have included the use of sulfur-containing dehydration agents during thermal polymerization, resulting in some higher Mw polysuccinimides of excellent light color (Batzel et al., 1996, U.S. Pat. No. 5,508,434). Another method employed mild crosslinking, sufficient to increase Mw, but insufficient to insolubilize the product polyaspartates that are produced from polysuccinimide via the aqueous, ring-opening procedure (Ross et al., 1996, U.S. Pat. No. 5,552,516).
In actual practice, prior art methods have generally produced polysuccinimides that range from about 10,000 to 30,000 in Mw, as measured by gel permeation chromatography (GPC) techniques. In addition, most studies have not addressed the molecular morphology of the products. Accordingly, there remains a need for a simple process for forming very high molecular weight, low color, substantially linear polysuccinimides.